A three-group zoom lens is known, and has often been used with various types of cameras where a compact size zoom lens having aberrations that are sufficiently well-corrected for many applications is desired. Recently there has been a rapid increase in consumer demand for digital cameras and video cameras. In such cameras a compact zoom lens having high-quality imaging with a low level of distortion is typically required. On the other hand, digital cameras and video cameras utilizing a solid-state image sensor device, such as a CCD array, must have the zoom lens designed to meet specified conditions not necessarily required in other applications.
Automatic focus has become almost a standard feature, and high-speed focusing is generally desired. Because of this, a zoom lens having an xe2x80x9cinner-focusxe2x80x9d format or a xe2x80x9crear-focusxe2x80x9d format is often used. These formats each provide a lightweight zoom lens with the lens elements being positioned near the camera body, thereby enabling the drive-barrel mechanism to be short and efficient. A three-group zoom lens design generally provides much better performance than a two-group zoom lens design and thus is usually preferred.
The present inventor has previously disclosed a design for a three-group zoom lens in Japanese Laid Open Patent Application No. H10-293253. The primary characteristic of the zoom lens disclosed therein is that the entrance aperture is purposefully positioned far from the detecting surface because prior art solid-state image sensor devices, such as CCD arrays, do not efficiently detect light rays unless they are incident almost normal to the detector surface. This is quite different from the case where a photographic film is used as the detector. Therefore, in order to image an object onto the detector surface of a CCD array, the beam of light needs to be radiated at nearly normal incidence to every image position. In other words, it was previously desirable to set the aperture stop (i.e., the radiating pupil) as far as possible from the detector surface (i.e., the image) in order to maximize the detection process.
Recently, a CCD has been developed which will accelerate the development of more compact optical systems. Namely, a CCD has been developed which absorbs light most efficiently when the aperture stop is positioned a limited distance from the image surface. As a result, acceptable absorption occurs where the distance from the aperture stop to the image surface is up to approximately five times the image diameter (where the image diameter is two times the maximum image height).
Concerning the newly developed CCD array, when the aperture stop is far from the image surface, so that the light is incident nearly normally, the efficiency of light absorption decreases. Thus, a three-group zoom lens which is designed to minimize shifting of position of the lens elements when changing magnification has a disadvantage as to this aspect, in that the design is not well-suited for the newly developed CCD array.
On the other hand, Japanese Laid Open Patent Application S59-31922 discloses a different type of zoom lens that uses a xe2x80x98rear-focusxe2x80x99 method with the traditional three-lens-group design. This zoom lens changes the magnification when focusing at a nearby object so that the position of the aperture stop (i.e., the radiating pupil) is situated nearer the imaging surface at the wide-angle end. When maintaining a fixed distance from the aperture stop to the imaging surface, the interval between the second lens group G2 and the third lens group G3 increases at the telephoto end when focusing to infinity. Therefore, the design goal of creating a more compact zoom lens is difficult to accomplish.
In addition, CCD arrays, which are of the type of detector commonly used for electronic still cameras, have rapidly evolved to provide higher density pixels as well as wider-angle views. Recently, a CCD array has been developed having over 200 million pixels. This development trend is expected to increase in the future. Because of this trend, higher resolution imaging will be required of the imaging lens used with this type of electronic still camera.
The present invention is a three-group zoom lens to be used with a solid-state image sensor device as in either a digital camera or a video camera. The zoom lens of the present invention has a zoom ratio of 2.5 as well as high-speed focusing. A first object of the invention is to provide a three-group zoom lens having an overall size that is compact. For example, the length from the most object-side surface of the three-group zoom lens to the image surface is less than 6 times the image diameter, where the image diameter is two times the maximum image height. A second object of the invention is to provide a zoom lens having aberrations that are well-corrected, thereby enabling the image to have a high resolution.